This previously resulted in an ICE:
cc65: Check failed: (Entry->Type->C & T_MASK_SIGN) == T_SIGN_SIGNED,
file 'cc65/symtab.c', line 874
This CHECK is in the code that deals with changing `int` bitfields to
`unsigned int`.
Work around this by treating enum bit-fields as having their signedness
specified, so this type change code does not get called.
Fixes#1244.
